(1) Field of the Invention
The present invention relates to the field of underwater acoustic sound detection through an ice layer. Specifically, a system and method of use is provided for the sound detection wherein the sound detection is accomplished by impinging on an ice object that is partially submerged. A Laser Doppler Vibrometer sensor measures surface velocity and therefore an acoustic pressure signal present at the ice surface being probed. Likewise, sensing of the underwater sound can be obtained by measuring the ice surface displacement using the sensor.
(2) Description of the Prior Art
The present invention provides a system that utilizes a Laser Doppler Vibrometer (LDV). The LDV is a commercial product that is based various interferometric system designs [See U.S. Pat. No. 5,838,439 and U.S. Pat. No. 5,883,715].
In operation, the LDV focuses a single output laser beam onto a measurement surface. The LDV then detects a laser beam reflected from the surface. By interfering the detected beam that was reflected by the measurement surface with a reference beam within the LDV; a measure of the surface velocity (and displacement) is obtained.
The technique of utilizing a LDV has been applied to measure water surface vibrations in order to remotely detect underwater sound since the water surface will vibrate when influenced by an acoustic pressure field. The underwater signals that are sensed can be used for active or passive sonar, surveillance purposes, or underwater acoustic communications from a number of submerged objects. Perpendicular incidence of the focused laser beam on the dynamic water surface waves is critical for obtaining a reflected signal.
Similarly, the laser beam must be reflected back from the ice surface for sensing to occur. However, measurement of ice surfaces is demonstratively different. The ice does not undergo dynamic wave motion and its surface reflectively is not consistent. The ice surface may not be smooth and may be covered with snow, each factor affecting the reflective properties. Back-reflection of the focused laser beam from the ice surface will occur when the laser beam is perpendicularly incident upon a smooth, specularly reflecting section of the ice surface.
Laser probing of ice surfaces provides intermittent laser returns thus causing optical and acoustic signal dropout at various points. Retro-reflective aids to sensing can be employed to mitigate signal dropout due to an ice surface reflectivity condition.
U.S. Pat. No. 6,320,665 (Ngoi et al.) discloses the use of a scanning laser interferometer to measure the motion of an object. The design of an interferometer is presented using an acousto-optic deflector to achieve scanning of the laser beam onto the target. The interferometer is then presented for measuring the relative height between two disk surfaces of a computer hard drive while the drive is spinning.
In the Ngoi reference, acousto-optic does not refer to the optical measurement of an acoustic signal and does not refer to detecting acoustic signals on an ice layer. Furthermore, the interferometer of the Ngoi design directs a reference laser beam onto a secondary surface in order to obtain a relative measurement of the disk drive surface motion. This differential interferometer was designed to measure the difference in heights between disk surfaces and not to detect absolute vibration velocity. Therefore, the design would not detect underwater sound from ice surfaces vibrations.
U.S. Pat. No. 7,113,447 (Matthews et al.) describes the use of a laser velocimeter to sense underwater acoustic signals by optically interrogating the outer surface of a hollow sphere mechanism (surrounded by a resilient matrix material, i.e., gel) that is in contact with the water environment. The sphere moves due to incident acoustic pressure waves. The motion of the sphere is measured using a laser velocimeter.
This technique necessitates using a sphere/matrix combination along with the laser velocimeter to detect underwater sound in water and would not be applicable for ice surfaces. The terminology for a laser velocimeter typically refers to a device used for measuring a flow velocity of a fluid.
U.S. Pat. No. 7,173,880 (Bernard) discloses a means of evaluating the velocity of sound wave propagation in the water so that an accurate determination of the depth of a pinging mechanism deployed by a surface vessel can be made. This is not a sensor to detect underwater sound signals and as such would not be applicable to detecting sound through an ice layer.
U.S. Pat. No. 7,251,196 (Antonelli) outlines a method and apparatus for detecting underwater sound via the vibrations caused by an impinging acoustic pressure wave on the water surface using a laser vibrometer in combination with a glint tracker device. The Antonelli reference contains an apparatus specific to detection on a dynamic water surface. The glint tracker's laser steering mechanism continually directs the sensing laser beam on the water surface where the surface slope provides a back-reflection to the remotely—located sensor apparatus. The ice surface does not move with the same dynamics as water surface waves. Detecting underwater sound through an ice layer would not involve tracking of water surface waves, but rather depends on the ice surface reflectivity.
Additionally, coupling of the acoustic energy from the water to the ice occurs. A portion of the sound wave energy is transmitted through the water-ice boundary as a function of the acoustic impedance difference between water and ice. This sound wave then propagates through the ice layer where it would be detected by the laser vibrometer sensor at the ice-air boundary. The propagation angle of the acoustic wave is increased (relative to the surface normal) at the water-ice boundary due to the increased sound speed in the ice relative to the water.
U.S. Pat. No. 7,283,426 (Grasso) outlines the use of a pulsed underwater laser system to detect submarine wakes. The laser beam pulses emitted into the sea reflect from moving particles in the water. By correlating the speckle pattern received by successive pulse an estimate of the degree of particle motion is obtained. A ship wake is suspected if the correlation indicates larger particle motion. In the cited reference there is no means of detecting the underwater sound, particularly on an ice layer. The Grasso reference outlines a non-acoustic sensor for measuring in-water particle motion from which ship wakes would be determined. Such a system would require a laser to propogate within a water column to detect the motion of in-water particles and thus would not be applicable to in-air detection of acoustic vibrations on ice surfaces.
U.S. Pat. No. 7,613,075 (Cray) and U.S. Pat. No. 7,259,864 (Antonelli, et al.) outline means of using a laser vibrometer to sense the inside hull surface of an underwater vehicle (such as a torpedo or a ship sonar dome). The laser vibrometer probes the inside surface of the underwater vehicle to measure vibrations that occur due to impinging acoustic pressure waves to detect the in-water sound. This technique could not be used to measure the underwater sound through an ice layer from a remote location because it is designed for short propogation distance to the inner hull surfaces of an underwater vehicle that can be properly coated for high optical reflectivity. The acoustic energy interaction with the hull surface is a critical piece to reference but would not apply to the optical sensing of ice surface vibrations.
United States Patent Application Serial No. 2004/0252587 (Melese) discloses a method of imaging surface vibrations using reflected light captured by a photodetector array. The method relies on amplitude fluctuations of the light on each element of the photodetector array and performs analysis (Fourier transform) on each photodetector array element signal output. The cited reference relies on a light source that illuminates the entire object to be imaged. The method does not rely on any interferometry and therefore does not measure phase changes in the light. Thus, the method lacks fine scale resolution of the surface vibration velocity that vibrometers offer. The method also does not have the sensitivity for detecting softer acoustic sounds that may propagate to an ice surface. The technique basically detects a vibration velocity on the order of 100 micrometers per second rather than sensitivity on the order of 0.01 micrometers per second, which is the current state of the art for laser vibrometers.
United States Patent Application Serial No. 2009/0201763 (Jones) outlines a system for detecting underwater objects (such as mines) using an in-air laser to generate in-water sound and a sonobuoy field to detect the sound reflected from underwater objects. The sonobuoys would then wirelessly communicate with a host system to process the acoustic signals detected by the sonobuoys. This system is not capable of being used to detect underwater sound on an ice surface.
The foremost issue governing LDV performance capabilities on uneven reflective surfaces is the signal dropout due to laser reflections not being captured by the sensor system. The accuracy and reliability of an in-air probe is dependent upon receiving laser reflections from the surface. The laser transmitter and receiver are typically co-located in interferometric-based vibrometer systems, and thus require a retro-reflection of the laser beam from the surface. Since uneven ice surfaces, and snow covered conditions prevail in realistic situations, significant intermittence of a received signal may be expected as these surface conditions disallow laser retro-reflection.